We are studying the regulation of erythrocyte pyruvate kinase by phosphorylation stimulated by c-AMP. We are studying potential interactions between the phosphorylation and dephosphorylation effects and those of the oxidation-reduction cycle previously described. The effects of fructose diphosphate on the regulatory processes and on the altered pyruvate kinase are also being studied. We have isolated and are purifying a protein phosphatase from the erythrocyte which reactivates pyruvate kinase by dephosphorylating it. The regulation of the phosphatase is expected to be a major study this year.